Abstract: The invention provides apparatus for forming images at a high speed and with small power consumption, in which a discharge lamp for exposure remains `on` and electric power supplied to the lamp is reduced to such an extent that the lamp is turned down during standby period for exposure. The invention also provides image forming apparatus including a temperature detector for detecting the temperature of an exposure lamp, plural filters, actuators for moving the filters in and out of the optical path, and a controller for regulating the actuators based on temperature data, thus forming stable images irrespective of the temperature of the lamp. The invention further provides image forming apparatus including a cooling fan for cooling an exposure lamp and lamp temperature controller for maintaining the temperature of the exposure lamp constant by activating the cooling fan based on temperature data detected by a lamp temperature detector, thus forming stable images of a desirable color or tone.